Remove backend.is_current assertion

[NathanFlurry]

Originally added in this commit, we can't always assert that backend.is_current is true after backend.make_current is called The issue seems to be an issue specifically on the later versions of macOS – likely specifically in Mojave. For more information, see this issue.

[tomaka]

If this hack is needed, I would prefer to put it in glutin instead.

[fschutt]

At least from what I can see in azul (since this check was removed in azuls version of glium) the check doesn't seem to do anything. The apps seem to run fine, even when backend.is_current evaluates to false. So I think it is just unnecessary and should be removed (although I'm not sure why it doesn't work specifically on Mojave, maybe there is some kind of buffering of driver commands going on).